1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, printer, facsimile machine, or the like, and in particular relates to an image forming apparatus using an intermediate transfer device.
2. Description of the Related Art
In image forming apparatuses such as copiers, printers, or facsimile machines, a transfer process is performed in which a toner image obtained by developing an electrostatic latent image formed on a photoreceptor (being a latent image carrier) is transferred to a recording medium such as a sheet.
In such a transfer process, there is a case of transferring a monochromatic image in which the toner image is directly transferred from the photoreceptor to the sheet. Otherwise, in a case of transferring a full color image using an intermediate transfer body, the color-separated toner images of each color formed on the photoreceptor are sequentially transferred to the intermediate transfer body in a primary transfer process, and subsequently the toner images of each color superposedly transferred on the intermediate transfer body are en bloc transferred to the sheet in a secondary transfer process.
A higher precision in the obtained image has been desired for both the monochromatic image and the full color image recently. As one approach to obtain the high quality in the formed image, image density is minutely controlled by detecting the density of the toner on the intermediate transfer body and by controlling the toner density in the developing device and the developing conditions. In the full color image formation, in particular, patterns on the intermediate transfer body are detected so as to control color shift in main scanning and sub-scanning directions, thereby obtaining a high quality image even in the superposedly obtained color image.
When detecting the toner density, the condition of the detection target surface may adversely affect the detection precision. For example, projections and scratches on the surface to be detected of the intermediate transfer body may cause detection errors.
In particular, in a case of using a movable belt as an intermediate transfer body on which visualized images formed on each of a plurality of latent image carriers are sequentially transferred in the primary transfer process, the superposedly transferred images are en bloc transferred to the recording sheet, and thereafter, the belt-shaped intermediate transfer body is cleaned by a cleaning device, when the toner density and the patterns on the belt are detected by any detection means, the detection output results may include variations due to fluttering or winding of the belt. Typically, the toner density or the patterns are detected at a position where the intermediate transfer belt is wound over rollers. In such a case, foreign particles on the inner surface of the belt or projections on the outer surface of the belt may result in unexpected detection outputs.
Accordingly, to detect the toner density on the intermediate transfer body precisely, JP-2008-241958-A discloses a technology and an image forming apparatus including a detector to detect at least one of a toner amount and a color shift amount in a reference image transferred onto the intermediate transfer belt by the transfer means, a correction unit to correct at least one of the toner amount and the color shift amount based on the detection result by the detector, and a support roller for detection contacting an inner side of the intermediate transfer belt and having a flat portion with a substantially constant diameter at its center and gradually reducing in size toward outside from the flat central portion, in which the detector detects the reference image at a surface opposite the portion where the detection support roller contacts the inner side of the belt. However, this disclosed art cannot improve the detection precision because the roller is formed of a metal and the belt slips along it too easily.